The present invention relates to an improved method and apparatus for electrically connecting and disconnecting contact pads or traces on a circuit board to and from external leads. More particularly, the present invention relates to improvements in positioning and utilizing self-regulating heaters to effect electrical connections and disconnections at circuit pads on a circuit board.
Although the invention has as its primary utility the simultaneous joining of a plurality of electrical leads to a plurality of circuit board contacts, it will be appreciated that the principles described herein are equally applicable to joining a single lead to a single contact. Moreover, although the description set forth below mentions solder as the joining material, it is to be understood that any fusible material, such as doped conductive plastic material, may be employed.
Soldering cable leads to circuit board contact pads can be achieved in a variety of prior art methods and apparatus. The least desirable of these is manually effecting each of the multiple solder terminations; that is, the resulting repetitive soldering operations are time-consuming and costly. In addition, close spacing between adjacent connection sites renders it likely that one or more of the manual soldering operations will result in solder bridges between one or more pairs of the adjacent sites.
There have been a number of prior art attempts to solve the aforementioned problems, examples of which may be found in U.S. Pat. No. 3,396,894 (Ellis), U.S. Pat. No. 3,719,981 (Steitz), U.S. Pat. No. 3,750,252 (Landman) and U.S. Pat. No. 4,484,704 (Grassauer et al). Typically, in these and other prior art soldering procedures for forming multiple solder joints simultaneously, a soldering tool is moved to the connection sites to deliver the necessary thermal energy over a large continuous area spanning all of the connection sites. Upon energization, the soldering tool heats up until it overshoots a control temperature before settling down to that temperature. The control temperature is typically chosen somewhat above the ideal soldering temperature in order to compensate for less than ideal thermal energy transfer. This approach to thermal energy delivery has a number of disadvantages. One such disadvantage is damage to components resulting from overheating. For example, the thermal overshoot inherent in the heating tool can damage components disposed between the connection sites within the area heated by the tool. In some cases the overshoot may cause damage to the polymeric materials, insulating materials and adhesives at the connection site.
Another prior art problem associated with the simultaneous soldering of sets of plural contacts relates to solder bridging between the connection sites. The pressure and thermal energy applied to spaces between the connection sites tends to cause the solder to run between those sites and form solder bridges. This problem has been addressed in some prior art apparatus such as that disclosed in the aforementioned Grassauer et al patent. In that apparatus the solder is sandwiched between two layers of polymeric material, one of which has window openings arranged to permit the solder, when melted, to flow through to respective connection sites. Barriers are provided between the windows to preclude solder bridging. While this technique, when properly employed, minimizes bridging, proper employment is hampered by the difficulty of aligning the windows (which must necessarily face downward and away from the technician) with respective contact pads at the connection sites. Care must also be taken to avoid displacement of the solder within the layered polymeric package prior to heating the apparatus in order to assure that solder is present at each of the window openings.
Connectors having terminals with solder tails extending therefrom for reception in plated through holes of a circuit board are well known. Relatively recently, in the interests of facilitating automated contact placement and economy of circuit board manufacture, surface mount connectors have been developed with terminals having solder tails formed for disposition against respective plated contact pads on the surface of the board. The terminals are typically fixed in the connector housing, and the tails are formed to sufficient length to assure compliance with the pad or pads, regardless of any tendency of the board to warp. Typically, each solder tail is individually manually soldered to a respective contact pad by any of a variety of known techniques. The repetitive soldering operations are both time-consuming and costly.
Another problem associated with prior art techniques for simultaneously joining multiple solder tails of a surface mount connector to respective contact pads on a printed circuit board relates to positionally aligning of the components during soldering. It is not only necessary for the multiple solder tails to be positionally aligned with respective multiple contact pads on the printed circuit board surface; in addition, the solder tails and contact pads must be simultaneously aligned with multiple respective sections of the solder delivery unit. The resulting alignment procedure is often unwieldy at best.
It is desirable, therefore, to provide a method and apparatus for electrically joining contact pads of a circuit board to respective leads in a cable or solder tails of a surface mount connector without a third component to be aligned at each connection site. In addition, it is desirable that the thermal energy required to melt the fusible material (e.g., solder) be available virtually instantaneously after energization of the heater, and that the heater be arranged to provide no more thermal energy than is required to melt the solder at the various connection sites. It is also desirable that the heater be simple and convenient to employ and energize, and that it have a relatively low mass in order that the heater may cool quickly after a soldering operation. It is likewise desirable that the method and apparatus provide the thermal energy to a localized area in which the connection sites are located.
The present invention makes use of a relatively new automatic self-regulating heater technology disclosed in U.S. Pat. No. 4,256,945 (Carter et al), U.S. Pat. No. 4,623,401 (Derbyshire et al), U.S. Pat. No. 4,659,912 (Derbyshire), U.S. Pat. No. 4,695,713 (Krumme), U.S. Pat. No. 4,701,587 (Carter et al), U.S. Pat. No. 4,717,814 (Krumme) and U.S. Pat. No. 4,745,264 (Carter). The disclosures in these patents are expressly incorporated herein by reference for purposes of background information. A heater constructed in accordance with that technology, referred to herein as a self-regulating heater, employs a substrate of copper, copper alloy, or other material of low electrical resistivity, negligible magnetic permeability and high thermal conductivity. A thin layer of thermally-conductive magnetic material is deposited on all or part of one surface of the substrate, the layer material typically being an iron, nickel or nickel-iron alloy, or the like, having a much higher electrical resistance and magnetic permeability than the substrate material. The thickness of the layer is approximately one skin depth, based on the frequency of the energizing current and the permeability and resistance of the layer. A constant amplitude, high frequency alternating energizing current is passed through the heater and, as a result of the skin effect phenomenon, is initially concentrated in one skin depth corresponding to the thickness of the magnetic surface layer material When the temperature at any point along the heater reaches the Curie temperature of the magnetic material, the magnetic permeability of the magnetic material at that point decreases dramatically, thereby significantly increasing the skin depth so that the current density profile expands into the non-magnetic substrate of low resistivity. The overall result is a lower resistance and lesser heat dissipation. If thermal sinks or loads are placed in contact with the heater at different locations along the heater length, thermal energy is transferred to the loads at those locations with the result that the temperature does not rise to the alloy Curie temperature as quickly at those locations as it does in the non-load locations. The constant amplitude current remains concentrated in the higher resistance alloy layer at the load locations which dissipate considerably more resistive heating energy than it dissipated in the non-load locations where the current is distributed in the low resistance substrate.
The aforementioned self-regulating heater technology is described in connection with soldering a in co-pending U.S. Patent Applications Ser. No. 277,116 filed by McKee, et al. and entitled "Self Regulating Temperature Heater with Thermally Conductive Extensions"; and U.S. Patent Application Ser. No. 277,362 filed by McKee, et al. and entitled "Surface Mount Technology Breakaway Self Regulating Temperature Heater"; both filed concurrently herewith and owned by the same assignee as the present patent application. The disclosures in both of those patent applications are expressly incorporated herein by reference. In patent application Ser. No. 277,116 a self-regulating heater is disclosed as having a plurality of thermally and electrically conductive connecting members formed integrally therewith and adapted to be disposed, at respective connection sites, between circuit board contact pads and leads to be soldered to that pad. The connecting members are severed from the heater assembly after a soldering operation, and remain permanent components of each connection site. In patent application Ser. No. 277,362 a self-regulating heater has the terminals of a surface mount connector formed integrally therewith, the terminals being configured to have solder tail portions adapted to contact respective pads on a circuit board. After a soldering operation the terminals are severed from the heater. Although the heaters disclosed in these two patent applications function well for their intended purposes, neither is conveniently useful for applying thermal energy to unsolder or remove connections from a circuit board.
It is therefore desirable to provide a method and apparatus for connecting and disconnecting circuit board contact pads to external leads and terminals whereby a self-regulating heater may be employed for both connecting (i.e., soldering) and disconnecting (i.e., unsoldering) applications. Moreover, it is desirable to provide a method and apparatus for effecting electrical connections and disconnections at circuit board contact pads wherein a heater for generating thermal energy to melt solder or other fusible material is a permanent part of the circuit board structure.